ADVANCED TOPICSCOMP2101Fall 2019

BASIC TOPICS

Jobs and Signals

Error Output

Functions

ERROR OUTPUT

Commands may generate unwanted or irrelevant error messages

That output can be saved as a log, sent to whoever should see it, or discarded

Logs are usually kept in /var/log for programs we care about managing

Redirecting error output with 2> or 2>> allows you to capture that output

Consider using the logger command to send messages to the system logging daemon - example uses process substitution to turn a sub-shell into a filename for redirection purposes

somecommand 2> >(logger -t $(basename "$0") -i -p user.warning)

FUNCTIONS

A function is a named script block, it creates a command you can use elsewhere in your script

Inside a function, the arguments variables ($1, $2, $3, etc.) contain whatever was on the command line that ran the function

Functions end with the status code of the last command to run in the function, data results can be passed back on stdout, or using an intermediary means such as storing data in a file or variable

Variables are by default shared between functions and the rest of the script

A function may be ended immediately with the return command

Function definitions can be viewed with the type command, and deleted with the unset command

function myFunction { list }

Lab 5 - firstfunctions.sh

SIGNALS

Signal are a way of notifying a process you want it to do something, usually terminate

Signals can be sent using the kill command TERM is default, INT, QUIT, HUP are alternates

Processes can catch and process or ignore most signals

KILL, STOP, CONT cannot be caught or ignored

Signals STOP and CONT are used to pause/resume processes(jobs)

Signal KILL is used to forcibly immediately terminate a process

$ kill -SIGNAL pid $ pkill -SIGNAL processname

BASH JOBS

When bash executes a command line, it is said to be running a job

Jobs usually run in the foreground until completion and then bash displays a new prompt to let the user know it is ready for the next job

A job can be run in the background by appending & to the command line

The current background job list can be viewed with the jobs command

Background jobs can be brought to the foreground using fg %jobnumber

^Z tells bash to pause a foreground job (called stopping a job) by sending it the STOP signal

bg %jobnumber can be used to continue running a stopped job in the background

^C tells bash to ask the foreground job to terminate itself, by sending it the INT signal

Exiting bash will cause it to send a hangup signal to any background jobs that shell still has running, which may cause them to exit

TRAP

In a shell script, catching signals is done with the trap command

trap can run a command when a signal is caught, functions are often useful for this

function cleanup { rm /tmp/mytemporaryfiles logger -t `basename "$0"` -i -p user.info -s “Cleaning up and aborting” exit 1 }

trap cleanup SIGHUP trap cleanup SIGTERM trap cleanup SIGINT

Lab 5 - countdown.sh

MISCELLANEOUS TOPICS

Unnamed Arguments

Dialog UI

DEBUGGING THE COMMAND LINE

You can watch how bash does command line evaluation if you use set -x

Don't forget to turn it off with set +x if you use this

You can run scripts using bash -x scriptfilename to see all command lines in the script get evaluated before they execute

You can put #!/bin/bash -x as the first line of your script to make it always run in debug mode

DIALOG BOXES

For more complex user interactions such as choosing files, selecting items from a list, or presenting graphics on text-only terminals, there is a the dialog command

dialog can ask for input/decisions or display information

dialog is useful when you are working on a terminal and want to present interactions in a more user-friendly way than just displaying text

e.g.userpicked=$(dialog --menu "choose one" 0 0 0 a 1 b 2 c 3 d 4 e 5 --output-fd 1)(for ((i=0;i<=100;i+=10)) do echo $i;sleep 1;done)|dialog --gauge "progress" 7 60;clearfoo=$(dialog --rangebox "Pick a value" 8 80 1 9 5 --output-fd 1);clear;echo "You chose $foo"

dialog's command line can be inscrutable

DECLARE

The declare command can be used to display things stored in process memory that we can use or to give bash rules for a variable

declare -a varname will cause bash to only store arrays in varnamedeclare -A varname will cause bash to only store associative arrays in varnamedeclare -i varname will cause bash to only store integers in varname

declare -x varname will cause bash to put varname in the environment declare +x varname will cause bash to take varname out of the environment

Declare can be used with functions

declare -i myvar myvar=$((16 * 32)) myvar="red"

declare -x VARNAME VARNAME=“Data” declare +x VARNAME

Show me your work to get the marks for it